Aluminum plates have conventionally been used as supports for lithographic printing plates. In order to obtain satisfactory adhesiveness of a light-sensitive layer to an aluminum support, and to impart a water retention property to non-image areas, the aluminum plate is required to be subjected to the so-called graining, i.e., a surface treatment for roughening the surface. The graining is an important step in the production of lithographic printing plates, since it has great influences on applicability of a support to a plate making process and on printing durability (press life) of the resulting printing plate in off-set printing.
Known processes for graining include mechanical graining processes, such as sandblasting, ball graining, wire graining, brush graining with a nylon brush and an abrasive-water slurry, etc.; a chemical graining process comprising etching a special aluminum alloy sheet with an alkali etching solution as disclosed in Japanese Patent Application (OPI) No. 61304/76 (the term "OPI" herein used means "unexamined published application"); a chemical graining process comprising etching a general aluminum plate (e.g., grade 1100) with a saturated aqueous solution of an aluminum salt of a mineral acid as disclosed in Japanese Patent Application (OPI) No. 31187/80, corresponding to U.S. Pat. No. 4,201,836; an electrochemical graining process as disclosed in Japanese Patent Application (OPI) Nos. 146234/79 and 28123/73; a combination of a mechanical graining process and an electrochemical graining process as disclosed in Japanese Patent Application (OPI) No. 123204/78; and a combination of a mechanical graining process and a chemical graining process using a saturated aqueous solution of an aluminum salt of a mineral acid as described in Japanese Patent Application (OPI) No. 55291/81 corresponding to U.S. Pat. No. 4,242,417. However, each of these conventional techniques has respective disadvantages as set forth below.
In the case of ball graining, which is a typical process for mechanical graining, there are problems in that high skills are required for selection of the kind of materials of balls and the kind of abrasives and for control of water in carrying out abrasion, and the like, and it is difficult to conduct the operations in a continuous production. In the case of wire graining, the roughness of the resulting aluminum plate is non-uniform. Brush graining, that is an improvement over these graining processes, generally provides only simple and shallow roughness, and the resulting printing plate has a short press life. Further, traces of a rotating brush unfavorably remain on the surface, the roughness undergoes orientation of the rotating brush, or stains are apt to be formed in non-image areas.
The chemical graining process disclosed in Japanese Patent Application (OPI) No. 61304/76 requires use of an aluminum alloy plate containing from 1.6 to 2.5% of manganese, and has, therefore, the disadvantage of difficulty in obtaining raw materials, and the disadvantage of formation of stains on prints, depending on printing conditions.
The aluminum plate having been chemically grained by etching with a saturated aqueous solution of an aluminum salt of a mineral acid as disclosed in Japanese Patent Application (OPI) No. 31187/80, corresponding to U.S. Pat. No. 4,201,836 was found to provide a lithographic printing plate having only short press life.
The electrochemical graining process is effective to form uniform roughness having a large average surface roughness, as compared with conventional mechanical graining processes, such as ball graining, brush graining, etc., by selecting proper electrolysis conditions. However, this process has a disadvantage of extremely narrow ranges of allowable conditions. More specifically, products having uniform performance properties with a narrow scatter can easily be obtained when various electrical conditions, such as the composition and temperature of an electrolytic solution, current density, and the like are maintained constant. However, since these electrolysis conditions are strictly limited, it is extremely difficult to carry out minute control of these conditions within appropriate ranges. Moreover, when surface roughening of the aluminum plate is effected only by electrochemical graining, there arises an economic problem in view of the high consumption of electric power which results in a larger proportion of electric power in the manufacturing costs.
Further, according to the combined process of brush graining and electrochemical graining as taught in Japanese Patent Application (OPI) No. 123204/78 and the combined process of brush graining and chemical graining as disclosed in Japanese Patent Application (OPI) No. 55291/81, corresponding to U.S. Pat. No. 4,242,417, it is possible to obtain a uniform surface roughness free from orientation and to minimize the consumption of electric power. There are problems, however, that brush graining with the same brush for a long period of time results in the failure of securing constant quality due to the wear of the brush used and that the operation should be suspended each time the brush has worn to a given extent, thus interfering with continuous production. In addition, stains are readily formed in the non-image areas presumably because of outlasting influences of brush graining.